High shunting sensitivity track circuits



Jan. 2, 1962 c, STAPLES 3,015,723

HIGH SHUNTING SENSITIVITY TRACK CIRCUITS Filed Oct. 28, 1959 ilmINVENTOR. Crawford E Staples BY 4 -W HIS ATTORNE') United States Patent3,015,723 HIGH SHUNTING SENSITIVITY TRACK CIRCUITS Crawford E. Staples,Edgewood, Pa., assignor to Westinghouse Air Brake Company, Wilmerding,Pa., a corporation of Pennsylvania Filed Oct. 28, 1959, Ser. No. 849,3728 Claims. (Cl. 246-34) My invention relates to railway track circuitapparatus.

and more particularly to railway track circuits providing high shuntingsensitivity.

Reliable operation of a track circuit of an insulated railway tracksection requires that a current source not only satisfactorily energizea track relay when the section is unoccupied but also requires that suchcurrent be effectively shunted away from the associated track relay whenthe section is occupied. It is known that a shunt effected through thewheels and axles of a train or car varies in its resistance due to thepresence, at times, of a thin coating or fi-lm of relatively highresistance on the rail and/ or wheel surfaces.

The problem of providing a satisfactory shunt is particularlycomplicated in automatic classification yards for various reasons. Insuch yards oily 'filrns are frequently encountered due to, for example,the drippings from freshly oiled wheel bearings, brine drippings fromstanding cars, etc. An oily film has a tendency to spread, to collectdirt, and to transfer from wheel to rail or vice versa depending on thetemperature. This film resistance may be of such nature as to form ahigh resistance path to the low voltage usually employed in the trackcircuit and such low voltage will normally not break down or puncturethe film.

The shunting problems are made more difficult in track circuits used forclassification yards since the tracksections employed are normally soshort that only one or two axles (and the included wheels) are in atrack section during a given occupancy time. Obviously the more axlespresent in a track section the higher the probability that at least oneaxle would provide a satisfactory shunt. Further the track circuits mustbe sensitive and fast in operation since a series of cars may befollowing one another in rather close succession and the command signalsfor each car must be initiated and transferred to the control centerexpeditiously and accurately.

The problem of providing a reliable shunting operation is furthercomplicated in classification yards since it has been found that carssubjected to sharp curves or retardation have a tendency to bounce androll particularly at switches, points, frogs, and rail joints, thusresulting in momentary losses of shunt. In order to overcome themomentary losses of shunt a short time delay in the reset of the trackcircuit must be provided. Thus, the track circuit desirably includes atrack relay having a quick shunting but slow pickup characteristics.

It has been found that the characteristics of conventional trackcircuits are such that the current flowing in the track relay variesconsiderably from a minimum operating value when the ballast resistanceis low to a maximum value when the ballast resistance is high. Thismakes it difficult to maintain the timing of the track relay at arelatively constant value, since the track relay when highly energizedpicks up quickly and when shunted releases rather slowly. It is knownthat greatly increased shunting sensitivity of a track circuit can beobtained by increasing the voltage impressed across rails since a highvoltage tends to break down the nonconductive film to ermit trackcircuit current to flow through the wheel and axle units. However, if ahigh voltage is steadily applied between the rails the track circuitpower input tends to become excessive, especially under wet weatherconditions when the ballast resistance is relatively low. Further, whendry weather conditions exist, a high volt= age impressed across railstends to over-energize the track relay as well as to create otherundesirable conditions, such as affecting the operation of adjacenttrack circuits.

Accordingly it is a principal object of my invention to provide a trackcircuit having a high shunting sensitivity.

It is another object of my invention to provide a track circuitincluding a relay having a fast release time and a slow pickup time.

It is another object of my invention to provide a track circuit capableof shunting through high resistance film while utilizing a minimum ofpower.

In the attainment of the foregoing objects I provide a track circuitincluding means for connecting an alternating current across one end ofthe track section. The track circuit further includes a saturating orsaturable and a nonsaturatiri g or nonsaturable transformer having theirprimary windings connected in series with one another across the trackrails at the other end of:the track section. The saturating transformeris adjusted to satu rate at the minimum ballast resistance expected toexist across the track rails for providing a substantially constantvoltage thereacross over a wide range of track voltages. Thus a changein the voltage across the track rails provides a greater percentagechange in the voltage appearing across said nonsaturating transformer.The output of the saturating transformer is rectified and is connectedthrough a first P-N-P transistor, which transistor operates as a switchmeans, to energize a track relay. The output of the nonsaturatingtransformer is rectified and connected to bias a second P-N-P transistorto be normally conducting. The unsaturating transformer also provides avoltage to a relatively large capacitor, and a relatively highresistance connected across said capacitor. The more positive terminalof the capacitor is connected to the base of the second P-N-Ptransistor. The biasing network of the first transistor is connected toinclude the emitter to collector path of the second transistor and whenthe second transistor is conducting the bias voltage applied to thefirst transistor is such to cause the first transistor to be conductingheavily. When the current flow through the second transistor isdecreased the biasing voltage applied to the first transistor is such asto cause the current through the first transistor to also decrease andthereby deenengize the track relay. When a high resistance shunt occursacross the track rails, the saturated transformer will be negligiblyaffected, however, the output of the nonsaturating transformer will dropcausing the capacitor to discharge relatively slowly through the highresistance. Since the base voltage of the second transistor will behigher than its emitter voltage, the emitter to collector path of thesecond transistor will become a high impedance causing the current flowthrough the first transistor to decrease and the track relay to bedeenergized for a period of time dependent on the discharge time of thecapacitor.

It should be understood at the outset that although P-N-P typetransistors are employed in the embodiment described, N-P-N typetransistors could likewise be used by proper reversal of the biasvoltage, and by providing suitable connections from the capacitor to theproper electrodes to cut off the second transistor.

Other objects and advantages of my invention will become apparent whenthe following description and the accompanying drawing in which the solefigure is a schematic diagram of the track circuit according to myinvention. I

Referring to the figure the reference characters 11a and 11b designaterails of a railway track section 10 insulated by rail joints 13a and 13bfrom adjacent track sections. Alternating current to the track sectionis supplied from an alternating current source 9 of any suitable type,say a 115 volt 60-cycle source, through a transformer 1'5 having primaryand secondary windings 17 and 19. One terminal of the secondary winding19 is connected through lead 22 to track rail 11a at one end, theleft-hand end as oriented in the figure, oftrack section 10* and theother terminal of secondary winding 19 is connected through lead 24 andan adjustable resistor 21 to track rail 1112 also at the left-hand endof track section 10.

The usual ballast resistance existing across track rails 11a and 11b isindicated by the dotted resistors 14 connecting the two rails. As isknown, the ballast resistance may vary from a minimum to a maximumdepending upon moisture, temperature, etc. A shunt resistance on thetrack section 10 is indicated by the wheel and axle unit 16.

Current energy is taken from the track rails 11a and 11b by a pair oftransformers 2'5 and 31; transformer 25 being of a nonsaturable ornonsatur'atingtype and transformer 31 being of a saturable or saturatingtype. Instead of a pair of transformers, a single transformer having apair of output windings, one winding providing a relatively constantoutput and the other a variable output as, for example, two outputwindings wound on an iron core with one winding having no air gap andthe other winding having an air gap, might be used. One terminal of theprimary winding 23 of nonsaturable transformer 25 is connected throughlead 16 to one terminal of the primary winding 29 of saturabletransformer 31. The other or second terminal of primary winding 23 isconnected through lead 18 to track rail 11b at a point on the righthandend of track section 10; and the other or second terminal of primarywinding 29 is connected through lead 20 to rail 11a at a point also atthe right-hand end of track section 10. The primary windings 23 and 29are thus connected in series across the track rails 11a and 11b forpurposes to be described hereinbelow. The secondary winding 33 oftransformer 31 is' connected across the alternating current terminals 32and 3 4- of a full-wave diode rectifier 35 of the well known type.Thesecondary winding27 of transformer 25 is connected across the alternating .current terminals 40 and 42 of a full-wave diode rectifier 37which is similar to rectifier 35.

The positivedirect current terminal 36 of rectifier 35 is connected inseries through a diode 39 to provide a positive voltage to the emitter41 of a P-N-P type transistor 43. Diode 39 provides a substantiallyconstant voltage drop thereacross irrespective of the current flowingthrough transistor 43, A filtering capacitor 49 is connected in parallelto diode 39 to dampen any voltage variations across the diode. Thejunction of emitter 41, diode 39 and capacitor 49 is connected throughresistor 51 to the negative terminal 38 of rectifier 35. Transistor 43includes, in addition to emitter 41, a base 45 and a collector 47. Thecollector 47 is connected through an adjustable resistor 53 to theoperating coil of a track relay 5 and thence to the negative potentialterminal 38 of rectifier 3'5. Collector 47 thus provides the operatingcurrent to relay 55 to actuate relay contacts a. Resistor 53 permits aninitial fine adjustment of the voltage applied to relay 55.

As is known, varistor 57 changes its resistance dependcut on the voltageexisting thereacross. When transistor 43 is cut ofi an inductive kick isdeveloped in the operating coil of relay 55. This inductive kick isdissipated in the circuit including varistor 57, resistor '53 and coilor relay 55; thus protecting transistor 43 from any high voltage surges.

The positive terminal 36 of rectifier 35 is also connected seriallythrough a thermistor 63, a resistor 65 and a resistor 61 to the base oftransistor '43 to provide a voltage to base 45 which voltage is lesspositive than the voltage applied to emitter 41. Transistor 43 is thusbiased to be normally conducting, and in fact in one embodiment, thecircuit components are chosen such that transistor 43 is conductingrelatively heavy current. As is known,

thermistor 63 has a negative temperature coefiicient to compensate fortemperature variations, and thus tends to provide a relatively moreconstant emitter 41 to .base 45 bias. A capacitor 59 is connected inparallel to thermistor 63 and resistor 65 for purposes describedhereinbelow.

As noted, the alternating current terminals 40 and 42 of rectifier 37are connected across non-saturable transformer 25. A filter circuitcomprising inductor 66 connected in series with direct current terminal44 and a capacitor '67 connected in series with inductor 66 and acrossdirect current terminals 44 and 46 provides a smooth direct currentoutput to bias transistor 81. A parallel network comprising a resistor69, a capacitor 71'and a variableresistor 73 are connected in that orderacross terminals 44 and 46 and in parallel with one another. Resistor 69serves as a load resistor; the function of capacitor 71 and resistor 73will be described hereinbelow. The emitter 83 of transistor 81 isconnected through a resistor 91 and a diode 75 to the positive terminal44. The base 79 of transistor 81 is connected through resistor 77 anddiode 75 to terminal 44. Base 79 is also Connected directly to one sideof capacitor 71. Resistor 77 provides a voltage drop between emitter 83and base 79 to forward bias transistor 81. Diode 75 is poled to becomenonconductive and essentially an open circuit when the potential acrosscapacitor 71 is more positive than the potential appearing acrossterminals 44 and 46 of rectifier 37. A filtering capacitor 87 isconnected in parallel with resistor 77 and tends to maintain a constantvoltage across resistor 77. A resistor 89 is connected in series betweenthe positive terminals of capacitor 71 and resistor 73.

Emitter 83 of transistor 81' is also connected through resistor 91, anda diode 93 to the junction of resistor 65, capacitor '59 and resistor61. The other terminal of resistor 61 is connected to the base 45 oftransistor 43. Diode 93 is poled to prevent any positive voltagescoupled to emitter 83 from rectifier 37 and/or capacitor 71 fromappearing on the base 45 of transistor 43. Collector of transistor 81 isconnected through a resistor 94 and lead 96 to the negative terminal 38of rectifier 35 which rectifier, as noted above also, provides thebiasing voltages for transistor 43.

A lightning arrester 95 is desirably connected across leads 18 and 20 toprotect the track circuitry from high voltage surges.

In operation, the voltage appearing across track rails 11a and 11b isadjusted by means of resistor 21 to provide a sufiicient voltage acrossthe'series connected primary windings 23. and 29 of transformers 25 and31, respectively, to saturate saturable transformer 31. As is known,when transformer 31 is saturated it will provide a practically constantoutput voltage under all ballast resistance and shunt resistanceconditions above a minimum operating level. The voltage output ofnonsaturable transform er 2 5 is a function of the difference betweenthe voltage existing across the track rails and the voltage existingacross the saturable transformer 31. Consequently, a variation in thevoltage existing across said track rails causes a proportionally largeroutput voltage variation across transformer 25. This is the case due tothe fact thatwhen a high resistance shunt is applied across the trackrails 11a and 11b, and although there be only a slight drop in the railvoltage, practically all the change i across the track rails 11a and11b, transistor 43 is forward biased to be conducting a rather heavycurrent and track .relay 55 obtains normal operating or energizingcurrent, and the relay contacts a are picked up. As noted, diode 39 andresistor 51 are connected across terminals 36 and 38 of rectifier 35,and emitter 41 is connected to the junction of diode 39 and resistor 51.The voltage drop across diode 39 will be constant and relatively low;thus the voltage impressed on emitter 41 will be approximately one-halfvolt lower than the voltage present at terminal 36. Thermistor 63,resistor 65, diode 9-3, resistor 91, the emitter 83 to collector path 85of transistor 81, and resistor 94 are connected across terminals 36 and38 of'rectifier 35. .When transistor 81 is conductingheavily the circuitjust traced provides a voltage divider network which includes arelatively low emitter 83 to collector 85 impedance for providing apositive voltage to base 45; The voltage on base 45 will be a portion ofthe voltage developed across terminals 36 and 38 and will be somewhatless positive than the voltage on emitter 41. Since transistor 43 is ofthe P-N-P type, when the emitter 41 is more positive than base 45,transistor 43 will conduct rather heavily.

However, as is well known, when transistor 81 is either barelyconducting or cut off the emitter 83 to collector 85 impedance will berelatively large. The voltage divider network for base 45, previouslytraced above, will now include the high impedance of the emitter tocollector path of transistor 81 and the voltage applied to base 45willtend to be closer to'the voltage on terminal 36. The voltage onemitter 41 will be approximately one-half volt lower than the voltageappearing on the terminal 36 due to the drop across diode 39. n thiscondition transistor 43 will be reversed biased, that is, base 45 willbe more positive than emitter 41 and current flow through transistorwill be decreased or cut ofi.

The biasing voltages for transistor 81 which transistor in turn controlsthe operation of transistor 43 are as follows. Under normal conditionswith the track circuit not shunted, a positive voltage is applied fromthe positive terminal 44 of rectifier 37 through diode 75 and resistor91 to emitter 83, and through the diode 75 and resistor 77 to base 79.The upper terminal, as oriented in the drawing, of resistor 77 isconnected to the junction of diode 75 and resistor 91, while its lowerterminal is connected to base 79. Due to the voltage drop acrossresistor 77 the voltage applied to base 79 is less positive than thevoltage applied to emitter 83. Transistor 81 is thus forward biased andis normally conducting.

As noted above, capacitor 71 is charged to a voltage somewhat lower thanthe voltage across terminals 44 and 46 through the voltage droppingresistor 77. When the track rails are shunted, the voltage appearingacross transformer 25 and across rectifier terminals 44, 46 will dropcausing the voltage on emitter 83 to also drop.

However, the voltage across capacitor 71 changes relatively slowly sincecapacitor 71 which is relatively large must discharge through resistor89 and relatively high resistance resistor 73. Diode 75 is poled toassure the discharge path of capacitor 71 is through resistor 73.Typical values for capacitor 71 and resistor 73 as used in oneembodiment of the invention are 500 mfd. and 100K ohms, respectively.Thus, for a definite period of time, determined by the discharge time ofcapacitor 71, the voltage appearing on base 79 will be more positivethan the voltage appearing on emitter 83. In other words, the voltage onemitter 83 drops to a low value quickly, while the voltage on the base79 will drop relatively slowly. Consequently, the transistor 81 will bereversed biased during the foregoing period of time and current flowtherethrough will be decreased causing its emitter 83 to collector 85path to become a high impedance. discussed above, when transistor 81 hasa decreased current flow there'through transistor 43 will also have adecreased current flow therethrough or may be cut otf. In

any case, the current flow through the operating coil of track relay 55will be decreased sufficiently to deenergize the relay. A time delay fortrack relay 55 is provided by capacitor 59. With transistor 81 barelyconducting or cut off the lower terminal, as oriented in the drawing, ofcapacitor 59 is approximately at the positive potential of terminal 36of rectifier 35. tle or no voltage exists across capacitor 59. As notedabove, when transistor 81 begins: to conduct more heavily, the impedanceof the emitter 83 to collector 85 path decreases. The voltage at thejunction point of resistor 65, capacitor 59, diode 93 and resistor 61will now tend to decrease in potential due to the voltage dividingaction of the circuit traced-from terminal 36, thermistor 63, resistor65, diode 93, resistor 91, emitter 83 to collector 85, and resistor 94to terminal 38. However, before the foregoing junction point candecrease in potential, capacitor 59 which as noted above had little orno voltage appearing thereacross must now charge to the diiference involtage existing between terminal 38 and the junction point. Capacitor59 therefore charges through the circuit traced from its lower plate,diode 93, resistor 91, transistor. 81 and resistor 94 to terminal 38.Thus the voltage'ditferential or bias between emitter 41 and base 45will depend on the charging time of capacitor 59 through theforegoingcircuit, and the conduction of transistor 43 and the flow ofcurrent through relay 55 will be delayed.

Thetime delay provided by capacitor 59 is in the order of 0.1 second.The inherent time delay in the coil of relay 55 is about 0.3 second sothat the total time delay measured from the time shunt 16 is removedfrom tracksection 10 until relay 55 contacts a pickup is approximately0.4 second. .Since the voltage appearing across the, saturabletransformer 29 is substantially constant for all ballast conditions, thepickup time of the track relay 55 remains practically constant for allballast conditions.

In order to hold the track relay 55 down indefinitely, the .shuntresistance must be low enough to reduce the voltage across the saturabletransformer 29 sufiiciently to reduce the current flow through trackrelay 55 to the relay release value. The shunt resistance which willhold track relay 55 permanently in the released or deenergized positionis, of course, lower than the minimum operating ballast resistance.However, a momentary shunt of higher resistance than the ballastresistance will deenergize the track relay for a period of time asdescribed above.

In practice it has been found that a moving wheel and axle unit enteringa track section initially provides a high resistance shunt. Therefore,my circuit is particularly useful as a high sensitivity circuit toquickly detect the presence of a train in the track section. Also, thefilm on the rail is normally found in spots so that my circuit adds tothe inherent delay time of the track relay to maintain the relaydcenergized while the moving wheel and axle unit passes over the filmspot.

Although I have herein shown and described only one form of apparatusembodying my invention, it will be understood that various changes andmodifications may be made therein within the scope of the appendedclaims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

l. Atrack circuit for a section of railway track having ballastresistance existing across the track rails comprising, in combination,means providing alternating current energy to said track rails, asaturable transformer and a nonsaturable transformer each having aprimary winding and a secondary Winding, the primary winding of saidsaturable transformer and the primary winding of said nonsaturabletransformer being connected in series with each other across said trackrails, said saturable transacross over a range of voltages appearingacross said That, is, at this instant litrespectively, a track relayhaving an operating coil, first and second P-N-P type transistors eachhaving base,

emitter and collector electrodes, means connecting the output current ofsaid second transistor toenergize said trackrelay coil, first and secondbiasing means connect ing the direct current terminals of said first andsecond rectifierstosaid first and second transistors respectively forbiasing said transistors to a first stat'eof normalcurrent'fiowtherethrou'gh, said first biasing-means including a-capacitor'having itstwoterminals connected to said-fi'rst rectifier to be charged to aportion of the direct current voltag'e'cxistingacross said firstrectifier, aresisti've ele' ment connected across said capacitor', adiode-connected in series intermediate said capacitor andsaidfirstjrecti fier'with a polarity to cause saidcapacitor' todischarge through said re'sis'tive'element, the terminal of capacitorwhich is positively charged being connected to the base of said firsttransistor, said first transistor being biased to a'second; state ofreduced current flow therethrough by the potential on said capacitorwhen the voltage across said first rectifier drops, said capacitormaintaining saidfirst transistor in said second state for' a perioddependent' o'n the time required by said capacitor to discharge throughsaid resistive element to a potentialless than that appearing? acrosssaidrectifier, saidse'cond biasing'means including a voltage dividernetwork having its ends connected across the direct current terminals ofsaid second rectifier, said divider network comprising impedanceelementsconnected in series with the emitter-to base path of said secondtransistor, said base of said second transistor being connected. to apoint on said network between the positive terminal of said rectifierand the emitter-to-basc path offsaidfirst transistor, said firsttransistor when insaid first conducting state forming an effectivelylow' imped ance element in said network for causing the voltage appliedto the base of said second transistor to be higher than the voltage onthe emitter of said second transistor to bias said second transistor tosaid first conducting state, and said first transistor when in saidsecond state forming an eitectively high impedance element in saidnetwork for causing the voltage applied to the base of said secondtransistor to bias said second transistor to a second state of reducedcurrent flow and thereby decrease current flow to said track relay coil.

2. A track circuit for a section of railway track having resistanceexisting across the track rails comprising, in combination, meansproviding alternating current energy to said rails, first and secondtransformer means for coupling energy from said rails, first and secondrectifying means being connected to rectify the output of said first andsecond transformer means respectively, a track relay having an operatingcoil, first and second transistors each having base, emitter andcollector electrodes, means connecting the output current of said secondtransistor to energize said track relay coil, first and second biasingmeans connecting said first and second rectifying means to said firstand second transistors respectively for biasing said transistors to afirst state of current flow therethrough, said first biasing meansincluding a capacitor having its two terminals connected to saidrectifying means so that said capacitor is charged to a portion of thevoltage existing across said first rectifying means, a resistive elementconnected across said capacitor, current blocking means connectedintermediate said capacitor and said rectifying means for causing thedischarge path for said capacitor to be throughsaid resistive element,one terminal of said capacitorbeing connected to the base of said firsttransistor, said first transistor being biased to a-second state ofreduced current flow therethrough by the potential on said capacitorwhen the voltage across said first rectifying means drops, saidcapacitormaintainingrsaid first transistor in said second state for a perioddependent on the discharge time of said capacitor through said resistiveele ment, said second biasing means including a voltage divider networkhaving its ends connected across said second rectifying means, saiddivider network comprising impedance elements connected in series withthe emitterto-basepath of said first transistor, the base of said secondtransistor being connected to a point intermediate the ends of saidnetwork, said'first transistor when in said I first conducting stateforming ,an etftectively low impedanceelement of said network forcausing the voltage applied to'the base of said second transistor tobias said" second transistor to 'said first conducting state, and saidfirst tran'sistorwhen in said second state forming an effecttivelyhighimpedance element in said network for causing the voltage applied tothe base of said secondtransistor to bias said second'transistortoavsecond conducting state of reduced current how and thereby decreasecurrentflow to said track relaycoil.

3. A trackcircuit'for a secti0n1of railway track having resistanceexisting across thetrack, rails comprising, in V combination, meansproviding alternating current energy to said rails,asaturable-transfor-mer and'a nonsaturable transformer eachhaving aprimary winding and a second- 1 arywinding; theprimary winding of saidsaturabletransformer and the primary winding of said nonsaturabletransformer being connected in series with each other across said trackrails,tsaid saturable transformer developing a substantially constantvoltage thereacross over a range-of voltages appearingracross said trackrails, said ncnsaturable transformer developing a voltage thereacrosswhich is the difference between the voltage existing across said trackrails and the voltage across said satura ble transformer whereby avoltage change across said track rails develops a proportionally largerchange across said: nonsaturable transformer, first and secondrectifiers being connected across said secondary windings of said nonsaturable and saturable transformers respectively for rectifying theoutputs of said transformers, a trackrelay having an operating coil,first and second transistors each having base, emitter and collectorelectrodes, means connecting the output current of said secondtransistor to energize said track relay coil, first and second biasingmeans connecting the outputs of said first and second rectifiers to saidfirst and second transistors respectively for biasing said transistorsto a first state of current flow therethrough, said first biasing meansincluding a capacitor having its terminals connected to said firstrectifier so that said capacitor is charged to a portion of the voltageexisting across said first rectifier, a' resistive element connectedacross said capacitor, current blocking means connected intermediatesaid capacitor and said first rectifier for causing the discharge pathfor said capacitor to be through said resistive element, one terminal ofsaid capacitor being connected to the base of said first transistor,said first transistor being biased to a second state of reduced currentflow therethrough by the potential on said capacitor when the voltageappearing across said first rectifier drops, said capacitor maintainingsaid first transistor in said second state for a period dependent on thedischarge time of said capacitor through said resistive element, saidsecond biasing means including a voltage divider network having its endsconnected across said second rectifier, said divider network comprisingimpedance elements connected in series with the emitter-tobase path ofsaid second transistor, said base of said second transistor beingconnected to a point on said network between a terminal of said secondrectifier and the emitter-to-base path of said first transistor, saidfirst transistor when in said first conducting state forming aneffectively low impedance element in said network for causing thevoltage applied to said base of said second transistor to bias saidsecond transistor to said first conducting state, and said firsttransistor when in said second state forming an effectively highimpedance element in said network for causing the voltage applied to thebase of said second transistor to bias said second transistor to asecond conducting state of reduced current flow and thereby decreasecurrent fiow to said track relay coil.

4. A track circuit for a section of railway track having resistanceexisting across the track rails, comprising, in combination, meansconnecting electrical energy to said rails for providing a voltageacross said trackrails which' is a function of the resistance existingacross said track rails, first and second means for coupling electricalenergy from said track rails to the remainder of said track circuit,said second coupling means providing a substantially constant voltageover a range of voltages appearing across said track rails, said firstcoupling means providing a voltage which is the diiference between thevoltage appearing across said track rails and the voltage provided bysaid second coupling means whereby a voltage change across said trackrails due to a change in the resistance across said track rails developsa proportionally larger voltage change across said first coupling means,a track relay, switch means for at times connecting said track relay toreceive energy from said second coupling means, means for controllingthe operation of said switch means, means for connecting saidcontrolling means to said first coupling means to receive energytherefrom, said controlling means being deenergized when the resistanceacross said track rails decreases, and said controlling means whendeenergized opening said switch means for a limited period of time todecrease current flow through said relay during said limited period oftime.

5. A track circuit for a section of railway track having resistanceexisting across the track rails comprising, means connecting electricalenergy to said track rails for providing a voltage across said trackrails which is a function of the resistance existing across said trackrails, a first and second impedance means connected in series with eachother across said track rails for developing voltages thereacross, saidfirst impedance means developing a substantially constant voltagethereacross over a range of voltages appearing across said track rails,said second impedance means developing a voltage thereacross which isthe difference between the voltage existing across said first impedancewhereby a voltage change across said track rails develops aproportionally larger change across said second impedance, a track relayhaving an operating coil, switch means for at times connecting saidtrack relay coil to receive energy from said first impedance means,means for controlling the operation of said switch means, saidcontrolling means including a capacitor and a resistive element, saidcapacitor being connected to said second impedance means to be chargedto a voltage which is a portion of the voltage appearing on said secondimpedance means, the voltage on said capacitor deenergizing saidcontrolling means when the energy appearing on said second impedancemeans decreases due to a decrease in the resistance across said trackrails, said controlling means being deenergized for a period dependenton the time required for said capacitor to discharge through saidresistive element to a voltage less than that on said second impedancemeans, and said controlling means when deenergized opening said switchmeans to thereby interrupt current flow through said track relay coilfor a limited period of time.

6. A track circuit for a section of railway track having resistanceexisting across the track rails comprising, means connecting electricalenergy to said track rails for providing a voltage across said trackrails which is a function of the resistance appearing across said track10 rails, first and second impedance means connected in series with eachother across said track rails for developing voltages thereacross, saidfirst impedance means developing a substantially constant voltagethereacross over a range of voltages appearing'across said track rails,said second impedance means developing a voltage thereacross which isthe difference between the voltage existing across said track rails andthe voltage across said first impedance whereby a voltage change acrosssaid track 'rails develops a proportionally larger change across saidsecond impedance, a track relay having an operating coil, first andsecond transistors each having base, emitter and collector electrodes,means connecting the output current of said first transistor to saidrelay operating coil for energizing said relay, first biasing meansconnecting the voltage developed across said first impedance means tobias said first transistor to be conducting a preset current, said firstbiasing means including the emitter to collector path of said secondtransistor, second biasing means connecting the voltage developed acrosssaid second impedance means to bias said second transistor tobe'conducting a preset current, said second biasing means including acapacitor connected to said second impedance means to be charged to aportion of the voltage developed across saidsecond impedance means, aresistive element'connected across said capacitor, current blockingmeans connected for causing said capacitor discharge path to be throughsaid resistive element, said second transistor being biased to conductless than the preset current when the voltage across said secondimpedance drops to a value less than the voltage on said capacitor, saidsecond transistor conducting less than the preset current for a timedependent on the discharge time of said capacitor through said resistiveelement, and said second transistor when conducting less than the presetcurrent changing said first biasing means to cause said first transistorto also conduct less than the preset current and thereby deenergize saidrelay coil.

7. A track circuit for a sectionof railway track having resistanceexisting across the track rails comprising, in combination, means forcoupling alternating current to said track rails for providing a voltageacross said track rails which is a function of the resistance appearingacross said track rails, a saturable transformer and a nonsaturabletransformer each having input and output windings, the input windings ofsaid transformer being connected in series with each other across saidtrack rails, first and second rectifier means for rectifying the outputof said output windings of said saturable and nonsaturable transformersrespectively, first and second transistors each having base, emitter andcollector electrodes, a track relay having an operating coil connectedto receive energizing current from said first transistor, a firstbiasing circuit connecting the rectified output of said saturabletransformer to bias said first transistor to be conducting a givennormal current, said first biasing circuit including a voltage dividingnetwork connected across said first rectifying means, said voltagedividing network including the emitter-to-base path of said secondtransistor, a second biasing circuit connecting the rectified output ofsaid nonsaturable transformer to bias said second transistor to beconducting a given normal current, said second biasing circuit includinga capacitor connected to said nonsaturable transformer rectifyingmeansto be charged to a portion of the voltage developed across saidnonsaturable transformer, a resistive element connected across saidcapacitor, current blocking means connected to cause the discharge pathof said capacitor to be through said resistive element, said secondtransistor being biased to conduct a less than normal current flowtherethrough by the voltage existing across said capacitor when thevoltage across said nonsaturable transformer drops due to a change inthe resistance'existing across said track rails, said second transistorconducting less than normal current for a time dependent on thedischarge time constant of said capacitor 1T through said-resistiveelement, and'said,secondtransistor when conducting less than normalcurrent effecting-a series high impedance element in said voltagedividing network and changing the bias-on said first transistor causingsaid first transistor to have less than normal current flow therethroughfor deenergizing saidrelay.

8-. A track circuit for a section of railway track having resistance.existing across the track rails comprising, in combination, means forcoupling alternating current togsaid track rails forv providing avoltage: across: said track rails which is a function of theresistanceappearing across said track rails, a sa-turablet transformerand a non-saturable.transformer each having, input and output windings,the input 'windingsof saidftransformerbeing, connected in series witheach other across said track.

rails, means for rectifying the outputz oi said output windings, firstand second transistors each having base,

emitter and collector electrodes, atrack, relay having.-

said first: biasing circuit including the emitter, to? base path of saidsecond transistor, a secondv biasing, circuit connecting the rectifiedoutput of said nonsaturable transformer to biassaidsecond transistor:tobe ;inznormally conducting states, said second biasing circuitmeans.including a capacitor connected to said nonsaturable.

transformer rectifying means to be-charged toapportion of the voltagedeveloped across said nonsaturableltransformer, a resistiveelement.connectedacross said;capacisecond state of decreased current flow,therethrough by; the voltage across saidcapacitor whenthe-voltageacross: a said nonsaturable transformer drops due, to achange' in theresistance appearing across said-track rails, said sec ond transistorremaining in said secondvstate for. a time. dependent on-the dischargetime constant of saidcapaci-- tor through said resistive element, saidsecondv transistor when in said secondstate'opening said first biasing.

circuit for changing the bias on said first transistors cause ingsaidfirst transistor to. a state of. decreasedrcurrent flow. therethroughfor deenergi zingasaidv relay.-

Suits: Aug. 8, 193-3] Place. c. Nov. 5,719.40,

